1. Field of the Invention
When the energy of an oil and gas reservoir is partially depleted, gas lift techniques are frequently used to raise the formation fluids to the well surface. The theory and design of various gas lift installations is fully explained in GAS LIFT THEORY AND PRACTICE by Kermit E. Brown, published in 1967 by Prentice-Hall, Inc. Intermittent gas lift of liquid slugs of formation fluid is theoretically a very efficient method of secondary recovery. However, the gas, propelling the liquid slug up the tubing string, will penetrate the liquid slug and allow the liquid to fall back in the tubing as droplets or film on the tubing wall. One method commonly used to improve the efficiency of intermittent gas lift and reduce fall back is to install a plunger to separate the liquid slug from the propelling gas.
Free piston or plunger pumping apparatus has been used in the petroleum industry for many years. The plunger or free piston is propelled by gas through a production tubing string communicating with an underground hydrocarbon formation to the well surface, pushing a liquid slug ahead of it. The propelling gas may be supplied from the formation or by injecting gas from the well surface to a location intermediate the production tubing string.
2. Description of the Prior Art
Plunger lift or free piston pumping systems commonly have a vertical section of pipe and associated fittings called a lubricator forming a part of a wellhead. A string of production tubing extends below the wellhead to an underground hydrocarbon formation or producing zone with the lubricator and production tubing having a common bore. The wellhead has various outlets for communicating fluids.
The plunger generally has a longitudinal opening and a valve designed to open and closes the opening. Preferably, the valve is operated by a bumper at the top of the lubricator and a similar bumper supported within the tubing string near the lower end thereof. When the valve is open, gas and liquid can freely pass through the plunger allowing the plunger to fall through the production tubing until it contacts the lower bumper and the valve is shut. Gas pressure, injected into the tubing from the annulus between the tubing and casing, forces the piston up and the tubing lifting a slug of liquid. Various devices have been used to catch the plunger within the lubricator while formation fluids accumulate in the production tubing to form another slug and to control the injection of gas into the annulus between the tubing and casing and from the annulus into the tubing below the plunger.
U.S. Pat. No. 3,095,819 to Norman F. Brown discloses a free piston pumping system having a controller which responds to the presence of the plunger in the lubricator to open and shut a valve in the outlet from the wellhead and to actuate a catcher to trap the plunger within the lubricator.
U.S. Pat. No. 3,031,971 to Erskine E. Roach discloses a system similar to U.S. Pat. No. 3,095,819 which has a magnetic actuating device to sense the presence of the plunger within the lubricator.
U.S. Pat. No. 3,351,021 to E. K. Moore, Jr. discloses a system similar to the two above systems having an improved pneumatic shock absorber in the lubricator to arrest the plunger movement and a pneumatic sensor to detect the presence of the plunger within the lubricator. The pneumatic sensor triggers a mechanical timer which controls the outlet valve and a catcher which releases the plunger from the lubricator.
None of the above patents disclose the use of a plunger catcher and trip assembly which is actuated by the same controller that controls the injection of gas into the annulus between the production tubing and casing. None of the above patents disclose using the controller in the gas supply line to control the plunger. Rather, the above prior art systems rely upon the plunger to actuate the various components of each system.
A technical manual prepared in 1956 by Harold Brown Company, Houston, Texas, on page 25 discloses the use of an H. B. Type B-1 controller in the gas supply line to regulate the injection of gas into the production tubing string below the plunger. However, the B-1 controller is actuated by a magnetic switch which senses the presence of the plunger within the lubricator.